1. Field of the Invention
This invention pertains to dimming gas discharge lamps and, more particularly, to dimming compact fluorescent lamps.
2. Description of the Related Art
A gas discharge lamp converts electrical energy into visible energy with high efficiency. A gas discharge lamp is generally an elongated gas-filled (usually low pressure mercury vapor) tube having electrodes at each end. Each electrode is typically formed from a resistive filament(usually tungsten) coated with a thermionically emissive material, such as a mixture of alkaline earth oxides.
The steady-state operation of a typical gas discharge lamp is as follows: Voltage is applied across the resistive filaments, heating the electrodes to a temperature sufficient to cause thermionic emission of electrons into the discharge tube. A voltage applied between the electrodes accelerates the electrons toward the anode. Enroute to the anode, the electrons collide with gas atoms to produce positive ions and additional electrons, forming in the tube a gas plasma of positive and negative charge carriers. The electrons continue to stream toward the anode and the positive ions toward the cathode, sustaining an electric discharge in the tube and further heating the electrodes. (If the applied power is ac, the electrodes reverse polarity during each half cycle.)
The discharge causes the emission of radiation having a wavelength dependent upon the particular fill gas and the electrical parameters of the discharge. Because each collision produces additional electrons and ions, increases in the arc current cause the impedance of the lamp to decrease, a characteristic known as "negative resistance." Operation of the lamp is inherently unstable, due to this negative resistance characteristic, and current between the electrodes must be limited to avoid damaging the lamp.
Dimming of gas discharge lamps is well known. A circuit for dimming a fluorescent gas discharge lamp is disclosed in U.S. Pat. No. 3,927,345, issued Dec. 16, 1975, to Licata et al., incorporated herein by reference. A fluorescent lamp is a gas discharge lamp in which the inner surface of the tube is coated with a fluorescent phosphor. The phosphor is excited by ultraviolet radiation from the electric discharge and fluoresces, providing visible light. Licata discloses a phase control dimming circuit which provides phase controlled voltage from a 60 Hz ac source to a fluorescent lamp in series with an inductive ballast. The dimming circuit employs a bi-directional triode-type thyristor (triac) as the main switching device and includes a dc compensation circuit to ensure symmetrical triac firing delays in each half cycle of power flow from the ac source. There is no current through the lamp during the triac firing delay. Symmetrically firing the triac prevents an excessive amount of dc current from flowing through the lamp, which can cause lamp flickering and saturation of the inductive ballast. The circuit operates over a dimming range from about 100% to 50% of full light output. Below about 50% light output, the electric discharge cannot be sustained, because the triac firing delay is longer than the recombination time of the gas plasma in the discharge tube.
U.S. Pat. No. 4,001,637, issued Jan. 4, 1977, to Gray, discloses a gas discharge lamp dimming circuit that is capable of operating lamps at dimming levels below 50% of full light output. Low dimming levels are attained by providing uninterrupted filtered dc current to the lamp. The circuit includes a capacitive ballast, a full wave rectifier, and an output filter. Ac current flows from a phase controlled source through the capacitive ballast to a full-wave rectifier. Pulsating dc current is provided by the full-wave rectifier to an LC low pass filter and then to the lamp electrodes. However, dc current tends to cause anode oscillations, uneven illumination along the length of the lamp, and a reduction in useful lamp life.
Anode oscillations can be greatly reduced by operating a lamp at frequencies much higher than the fundamental frequency of the anode oscillation. U.S. Pat. No. 4,207,498, issued Jun. 10, 1980, to Spira et al., discloses a dimming system that includes a central inverter for providing a 23 kHz ac current through the lamp. The lamp can be dimmed over a range from 100% to 1% of full light output by adjusting the amplitude of the inverter output. The use of high-frequency ac current also may increase the efficacy of the lamp by as much as 20%.
At low light levels (less than about 30% of full light output), however, the lamp tends to "striate"; i.e., to break up into alternating bands of bright and dim areas along the length of the tube. The mechanisms that cause striation are not completely understood, but it is believed to result from high-frequency currents reinforcing a standing wave of varying charge distribution between the lamp electrodes. For reasons that are not clear, small-diameter lamps and lamps with sharp bends (typical characteristics of compact fluorescent lamps) are more likely to striate.